"Contact Hazard" from chemical agents can be defined in many ways depending upon specific scenarios. But in general, as applied to the hazard to human beings whose skin comes into contact with a surface that has previously been subjected to contamination by liquid agents such as nerve agents and mustard, it is defined as follows.
If a surface, e.g., paint on a substrate, that has previously been exposed to liquid agent, either is wiped so that no visible liquid remains, or is allowed to "weather" (stand undisturbed) until the surface appears to be completely dry, then contact hazard is that of the sorbed agent on and in the surface to humans touching or otherwise coming into direct contact with the surface.
Thus it is important to be able to detect the presence of agent residuals, and hence, hazards to humans, from surfaces in previously contaminated environments. Wiping the surfaces with detector paper will not work because sorbed agents will not be removed. Chemical analysis would require scraping, e.g., of paint samples followed by wet chemical procedures using trained personnel.